Portion of a lamp base



FIG. 1 is a perspective front view of a portion of a lamp base showing my new design;

FIG. 2 is a front elevation view thereof;

FIG. 3 is a left side view thereof; and,

FIG. 4 is a top plan view thereof.

The broken line showing of environmental structure is for illustrative purposes and does not form a part of the claimed design. 

The ornamental design for a portion of a lamp base, substantially as shown and described. 